The lead-acid battery in its various configurations is a time-honoured power source for diverse applications such as starting-lighting-ignition (SLI), uninterrupted power supply (UPS) and motive power. Continuous developments on the application side, for instance in the area of electric vehicles and hybrid electric vehicles (EV and HEV), impose challenging performance demands on battery technologies in general and lead acid batteries in particular. Pavlov summarized the relationship between battery specific energy in watt hours/kilogram (Wh/kg) and number of battery discharge/charge cycles for both flooded and valve-regulated type lead acid batteries. For both battery types, the higher the battery specific energy the lower the number of discharge/charge cycles and hence, the battery cycle life. Typically, a flooded battery with a specific energy of 40 Wh/kg can be used for about 500 discharge/charge cycles, while a battery producing only 30 Wh/kg can be employed for about 850 cycles. Thus, there is clearly a need to improve both the specific energy and cycle life of lead-acid batteries in order to make them more suitable for electric traction applications.
It is well known that the low utilization efficiency of the active mass, especially on the positive electrode, in conjunction with the heavy weight of the lead current collectors, limits the actual specific energy of the lead-acid battery. The structure of the current collector plays an important role in determining the utilization efficiency of the positive active mass (PAM). During discharge, on the positive electrode, the structure of the current collector must allow for significant volume increase (e.g. molar ratio of PbSO4 to PbO2 is 1.88) while maintaining electrical contact with the active material and assuring ionic transport to the electroactive sites.
There are many examples in the prior art describing techniques to increase the specific energy output by improving the porosity and specific surface area of the lead compound based paste (active material) applied onto the battery current collector (or grid). For example, Stoilov et al in U.S. Pat. No. 5,332,634 states that “there is a need for making lead electrodes with a porous active mass, which has a large active surface area and which strengthens the electrical connection between the active mass and the grid. Such a porous lead electrode would lead to electrochemical cells and accumulators which produce more power per unit of weight and also present very low electrical resistance.”
Regarding improvements in the battery current collector structure, Czerwiński and Zelazowska have described the electrochemical behaviour of lead deposited on a non-metallic open pore substrate, namely reticulated vitreous carbon (RVC). These authors prepared small, 1 cm2 geometric area, collectors by electrodeposition for 10 minutes of Pb from an alkaline solution containing 20 g/l NaOH to produce the negative electrode and anodic oxidation to form lead dioxide (PbO2) on the positive electrode using a concentrated lead nitrate based solution (Pb(NO3)2). The amount of generated active material, Pb and PbO2, was small at about 19.3 mg and 22.3 mg, respectively. Consequently, if a battery had been assembled with the above described electrodes, the corresponding capacity would have been extremely low, in the range of 4.5 mAh, insufficient for practical use. Furthermore, the battery structure described by Czerwiński and Zelazowska is not rechargeable in sulfuric acid, which is the operational electrolyte of lead-acid batteries, since the recommended active material generation procedure required alkaline and nitrate based electrolyte. Therefore, this prior art proposes a technique to manufacture a lead-acid battery with a cycle life of one (i.e. one time use). Clearly, it was not envisaged to paste active materials onto the reticulated substrate in order to create a high capacity, rechargeable battery.
Das and Mondal suggested developing lead acid current collectors with thin layers of active materials deposited on lightweight, electronically conducting substrates, such as a carbon rod. The rationale was only to reduce the ‘dead weight’ of the lead acid system, which would somewhat increase the specific energy.
Snaper, in U.S. Pat. No. 6,060,198 describes the use of reticulated metal structures for use as current collectors in batteries in which the reticulated structure consists of a plurality of pentagonally faced dodecahedrons. This prior art does not teach methods for using such a structure to improve the cycle life and performance of a lead acid battery and does not envisage the use of non-metallic electrically conductive substrates such as reticulated carbon to reduce battery weight. None of the above mentioned prior art references regarding reticulated structure suggest any need for combining the reticulated structure with a lead containing paste to create a rechargeable battery suitable for use in multiple charge/discharge cycles.